The Dynamics of Phenotypic Change and the Shrinking Sheep of St. Kilda

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Science  24 Jul 2009:
Vol. 325, Issue 5939, pp. 464-467
DOI: 10.1126/science.1173668

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Little Lambs

In changing environments, ecological and evolutionary dynamics are intimately intertwined. However, understanding the dynamics of phenotypic traits under natural conditions is still rudimentary. Ozgul et al. (p. 464; published online 2 July) dissected the dynamics of a phenotypic trait in the context of the contributing ecological and evolutionary processes. In a wild population of Soay sheep where mean body size has fluctuated substantially over the past 25 years and has, on average, gotten smaller, an ecological response to environmental variation is the major driver of the dynamics; evolutionary change has contributed relatively little: The sheep have become smaller because climate change has modified the way that density-dependence influences lamb growth rates.


Environmental change, including climate change, can cause rapid phenotypic change via both ecological and evolutionary processes. Because ecological and evolutionary dynamics are intimately linked, a major challenge is to identify their relative roles. We exactly decomposed the change in mean body weight in a free-living population of Soay sheep into all the processes that contribute to change. Ecological processes contribute most, with selection—the underpinning of adaptive evolution—explaining little of the observed phenotypic trend. Our results enable us to explain why selection has so little effect even though weight is heritable, and why environmental change has caused a decline in the body size of Soay sheep.

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